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What are those white lines in the sky?

If you’ve ever seen a jet fly across the sky, you may have noticed that there is sometimes a thin white line behind it. These clouds are contrails, short for condensation trails.

Water vapor is one of the byproducts of jet fuel combustion and will turn into ice crystals in the cold air at the high elevations where jet airplanes fly. Those ice crystals create a cloud (the contrail), which does not pose any public health risk.   

Contrails can evaporate quickly if humidity is low, or they can linger, and even grow by taking in water from the surrounding atmosphere, when humidity is high.

These contrails may appear to be grid-shaped, which is due to flight patterns generally following routes or corridors established by the National Airspace System (NAS). Just as there are highways for vehicles crisscrossing the United States, there are similar corridors in the sky most air travel follows.

You may sometimes hear contrails mistakenly called chemtrails. Occasionally, ADEQ is contacted by members of the public who wonder if these contrails might contain harmful chemicals or metals. All the data ADEQ has gathered from air, soil, and water across the state of Arizona do not show any evidence of this. 

Example of ADEQ Air Monitoring Data | View Data >

Aircraft emissions are regulated by the U.S. Environmental Protection Agency (EPA). Section 231 of the Clean Air Act (CAA) directs the EPA to establish aircraft engine emissions standards for any air pollutant that could “reasonably be anticipated to endanger public health or welfare.” The CAA further dictates in Section 232 that the federal Department of Transportation (DOT) is responsible for enforcing the standards established by EPA. DOT has delegated this enforcement responsibility to the Federal Aviation Administration (FAA). The FAA ensures compliance with these regulations by reviewing and approving certification test plans, procedures, test reports and engine emissions certification levels.

For a more in-depth look at contrails, see the EPA Aircraft Contrails Factsheet |  View Document >